Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo controller to control the velocity of the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 comprising a number of servo tracks 4 defined by servo sectors 60-6N recorded around the circumference of each servo track. Each servo sector 6i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6i further comprises groups of servo bursts 14 (A,B,C,D in the example shown), which are recorded with precise intervals and offsets relative to the track centerlines. The servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
Disk drives may employ a head having an inductive write element (coil) and a magnetoresistive (MR) read element. An MR read element may exhibit a non-linear response leading to asymmetrical positive/negative pulses in the read signal. Prior art techniques attempt to compensate for the MR asymmetry using a non-linear correction circuit implementing a second-order function:x+βx2.The coefficient β in the above equation may be adapted using a learning algorithm that attempts to minimize an error metric. However, the error metric is typically measured by evaluating the read signal samples after being filtered by a continuous-time filter as well as a discrete-time equalizer. The latency of the continuous-time filter and discrete-time equalizer introduces a transport delay into the closed-loop adaptation system that degrades performance and increases convergence time.